Next-generation typhoid vaccines are urgently required since Salmonella are rapidly developing resistance to antibiotics and current vaccines have safety concerns or are poorly immunogenic. The protective immunity induced by live vaccine strains (LVS) of Salmonella, or after resolution of primary typhoid via antibiotic treatment, are both very poorly understood. In this revised project, we propose to examine the development of CD4 memory T cells in these immunization models and define the critical requirements for protective immunity to secondary typhoid. This focus should allow for considerable synergy with Projects 1 and 4 where protective T cell and antibody responses to Listeria and Influenza are also being studied. The specific aims of this sub-project are: Aim 1. To determine the parameters of initial T cell activation and sustained antigen presentation that generate a robust Th1 memory response. Aim 2. To define the critical parameters of a protective secondary response to typhoid. Our preliminary data describe the development of state-of-the-art methods to track Salmonella-specific CD4 T cells in vivo, and the generation of a novel antibiotic-treatment model to study immunity to typhoid. Our hypothesis is that effective CD4 Th1 memory requires appropriate early T cell activation and sustained antigen presentation. It is also proposed that Salmonella-specific antibody responses can supplement a protective CD4 response during secondary typhoid challenge. This hypothesis will be tested in two specific aims using technology that allows detection of endogenous Salmonella-specific CD4 T cells and examination of acquired immunity to typhoid following antibiotic treatment. RELEVANCE (Seeinstructions):